Method for scrubbing flue gases

ABSTRACT

A method for scrubbing flue gases utilizing a spray tower for removing acidic gases and particulate matter from flue gases produced by processing operations of the type carried out in utility and industrial facilities. The spray tower is equipped with a tank that serves as a reservoir for an alkaline slurry used to remove acidic gases and particulate matter from the flue gases. The slurry is pumped from the tank to spraying devices located within the tower. The spray tower further includes an internal structure that enables the slurry to be oxidized and gently agitated within a limited region of the tank, and without the requirement for two separate aeration and agitation devices. As a result, the construction, operational and maintenance costs of the spray tower are significantly reduced as compared to prior art spray towers.

This is a divisional of application Ser. No. 08/580,693 filed on12/29/95 now U.S. Pat. No. 5,665,317.

This invention generally relates to gas-liquid contactors that use analkaline slurry to remove acidic gases from utility and industrialcombustion gases. More particularly, this invention is directed to agas-liquid contactor having an improved tank configuration that achievesmore efficient oxidization and agitation of the alkaline slurry.

BACKGROUND OF THE INVENTION

Absorbers are widely used to remove substances such as gases andparticulate matter from combustion or flue gases produced by utility andindustrial plants. Often of particular concern is sulfur dioxide (SO₂)and other acidic gases produced by the combustion of fossil fuels andvarious industrial operations. Such gases are known to be hazardous tothe environment, and therefore their emission into the atmosphere isclosely regulated by clean air statutes. Generally, these acidic gasesare removed with spray towers or other types of gas-liquid contactorsthrough the use of wet flue gas desulfurization (FGD) processes.

The cleansing action provided by gas-liquid contactors is generallyderived from the passage of flue gases upwardly through a towercountercurrently to a descending liquid that cleans the air. Wet fluegas desulfurization processes typically involve the use of an alkalinecleansing liquid, such as a calcium-based slurry or a sodium-based orammonia-based solution. As used herein, a slurry is a mixture of solidsand liquids in which the content of the solids can be any desired level,including the extreme condition in which the slurry is termed a moistsolid. Examples of calcium-based slurries are limestone (calciumcarbonate; CaCO₃) slurries and hydrated lime (calcium hydroxide;Ca(OH)₂) slurries formed by action of water on lime (calcium oxide;CaO). Intimate contact between the alkaline liquid and acidic gasespresent in the flue gases, such as sulfur dioxide, hydrogen chloride(HCI) and hydrogen fluoride (HF), result in the absorption of the acidicgases by the slurry. Thereafter, the liquid is accumulated in a tankwhere the absorbed acidic gases are reacted to form precipitates thatcan be collected for disposal or recycling. For example, in a flue gasdesulfurization process using a calcium-based slurry, the byproductprecipitate is gypsum (CaSO₄).

A known type of absorber 10 of the type using a spray tower 14 as agas-liquid contactor is shown in FIG. 1. The spray tower 14 is generallyan upright structure equipped with an inlet duct 12 through which fluegases enter the absorber 10. The inlet duct 12, as well as otherappropriate sections of the tower 14, are preferably formed from a highnickel alloy to promote their corrosion resistance. Above the inlet duct12 are banks 16 of spray headers 18 which introduce a spray of cleansingliquid, such as a calcium-based alkaline slurry, into the tower 14. Anynumber of banks 16 and spray headers 18 can be used as may be requiredfor a given application. One or more pumps 26 are required to recyclethe slurry to the spray headers 18 from a reservoir or tank 22 in whichthe slurry accumulates after contact with the flue gases. Each bank 16of spray headers 18 may be individually equipped with a pump 26 topromote the flexibility of the pumping and spraying operation toaccommodate varying demands by the scrubbing operation. After being"scrubbed," the flue gases are permitted to escape to the atmospherethrough a mist eliminator 24 at an upper end of the tower 14.

Intimate contact between the slurry spray and the flue gases rising thetower 14 results in the acidic gases being absorbed by the slurry, whichis then collected at the bottom of the tower 14 in the tank 22. Asindicated in FIG. 1, the tank 22 conventionally requires an aerator 28and one or more agitators 30. The aerator 28 injects anoxygen-containing gas, such as air, into the slurry accumulated in thetank 22 so that the slurry reacts with the absorbed acidic gases to formsolid precipitates, such as gypsum if a calcium-based slurry is used,that can be safely recycled or disposed. The agitators 30 are requiredto continuously mix the slurry in order to maintain the alkali and solidprecipitates in suspension. As shown, the tank 22 is equipped with anoverflow 20 to limit the level of slurry in the tank 22, and is adaptedto receive additional alkali 32 to compensate for that which has reactedwith the acidic gases.

As is typical, the agitators 30 are shown as fans. Though this type ofagitator is known to be effective, the gas bubbles generated by theaerator 28 are distributed throughout the slurry and lower its density,causing the slurry to occupy a considerable portion of the tower 14.From a structural standpoint, expansion of the slurry is disadvantageousbecause it necessitates that the height of the tower 14 be significantlygreater than would be otherwise necessary to accommodate a suitablequantity of slurry. From an operational standpoint, the high speed ofthe fan blade tips causes secondary nucleation of solids, resulting infiner precipitates that are more difficult to remove from the tank 22and more difficult to dewater and dry after removal. Furtherdisadvantages with the use of fan agitators 30 are the occurrence ofpump cavitation due to the intake of bubbles through the pump inlet 40in the tank 22, and the energy and costs to operate and maintain theagitators 30.

Those skilled in the art will appreciate that, in view of theconsiderations noted above, it would be desirable to reduceconstruction, operational and maintenance costs that are attributable tothe agitation of a slurry within the tank of a flue gas absorber.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a flue gas scrubbingapparatus for the removal of acidic gases from flue gases produced byutility and industrial facilities.

It is another object of this invention that the apparatus employs analkaline-based cleansing fluid for absorbing acidic gases from the fluegases, whereby oxidation of the cleansing fluid within an accumulationtank yields a precipitate that can be disposed of or recycled.

It is yet another object of this invention that the apparatus isequipped to agitate the cleansing fluid within the tank in a manner thatensures oxidation of the fluid while restricting agitation of the fluidto a relatively separate and limited region within the tank.

It is a further object of this invention to more fully and efficientlyuse the energy necessary to oxidize the cleansing fluid.

It is still a further object of this invention that the construction,operational and maintenance costs attributable to the agitation of thefluid within the tank are minimized.

The present invention provides a flue gas scrubbing apparatus forremoving gases from flue gases produced by processing operations of thetype carried out in utility and industrial plants. The apparatus isgenerally composed of a passage equipped with an inlet for introducingflue gases into the passage, and a tank that serves as a reservoir for acleansing fluid, such as an alkaline slurry, that is used to removeacidic gases from the flue gases. The tank is equipped to deliver thefluid accumulated in the tank to devices that introduce the fluid intothe passage, such that the fluid absorbs acidic gases within the fluegases. Thereafter, the fluid returns to the tank, which is furtherequipped with a device for injecting an oxygen-containing gas into thefluid. The injection of oxygen into the fluid causes the alkali to reactwith the acidic gases absorbed in the fluid from the flue gases. Forexample, in the case of desulfurization where an alkaline slurry of limeor limestone is used to absorb sulfur dioxide, oxygen or air is injectedinto the slurry to oxidize aqueous sulfite (SO₃ ⁻) into sulfate (SO₄ ⁻),the latter of which then reacts with calcium ions provided by the alkalito form gypsum.

According to this invention, the tank is further equipped with apartition that causes the fluid to circulate up through a first regiondelineated within the tank, and then down through a second regiondelineated within the tank, so as to agitate the alkali-containingfluid. More specifically, the partition operates in combination with theinjector to agitate the fluid within the first region within the tank,and aids in the chemical reaction between the alkali and the acidicgases while maintaining the remaining alkali and resulting solidprecipitates in suspension. Unique to this invention, agitation is notachieved with a fan, but instead relies on the agitation action achievedas the oxygen-containing gas is injected into the tank. The partitionadvantageously restricts frothing and expansion of the fluid within thefirst region, and therefore isolates the agitated fluid from the pumpand any supplemental alkali added to the fluid.

According to the above, agitation and oxidation of thealkaline-containing fluid are combined into a single unit thatefficiently performs both functions within the tank, and thereby yieldssignificant advantages over the prior art. For example, agitation is farless rigorous than that of fan agitators of the prior art, yet has beenfound to achieve a desirable agitation level for the gas scrubbingoperation. In addition, a more favorable environment for the reactionchemistry is achieved within the tank. For example, gentler agitationreduces the tendency for secondary nucleation of precipitates andpromotes the formation of larger precipitate crystals that can be morereadily dewatered, such that drying costs for the precipitates (e.g.,gypsum) are reduced. Also reduced is the occurrence of sulfite bindingof any alkali introduced directly into the tank, by which aqueoussulfite precipitates as calcium sulfite on the high pH surface of thealkali thereby lowering its dissolution rate and negatively affectingscrubber performance. Reduced sulfite binding is achieved becauseoxidation is restricted to a limited region of the tank away from thepoint at which additional alkali is introduced. If a pump is used toreturn the slurry to the passage, restricting agitation to a limitedregion within the tank also serves to isolate the pump from bubbleswithin the agitated fluid, thereby avoiding cavitation that can shortenthe life of the pump. Furthermore, the combination of oxidization andagitation of the fluid with a single unit, rather than two or moreseparate devices as done in the prior art, reduces the construction,operational and maintenance costs of a gas-liquid contactor. Inparticular, the capital and operating costs of conventional agitatorequipment are completely avoided.

Other objects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages of this invention will become moreapparent from the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 shows in cross-section an absorber of a type known in the priorart;

FIG. 2 shows in cross-section an absorber in accordance with a firstembodiment of this invention; and

FIG. 3 shows in cross-section an absorber in accordance with a secondembodiment of this invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 illustrates a flue gas scrubber in the form of an absorber 110configured in accordance with the teachings of the present invention. Asillustrated, the absorber 110 is generally similar to that of the priorart absorber 10 shown in FIG. 1, including a tank 122 in which acleansing fluid used to absorb gases from flue gas is accumulated.However, in accordance with this invention, the absorber 110 includes aninternal structure that enables the fluid to be oxidized and agitatedwithin a limited region of the tank 122, and without the requirement forseparate aeration and agitation devices, such as the aerator 28 andagitators 30 shown in FIG. 1. As a result, the construction, operationaland maintenance costs of the absorber 10 are significantly reduced ascompared to the prior art absorber 10. While the absorber 110 isillustrated as being of a particular construction, those skilled in theart will recognize that the teachings of this invention can be readilyapplied to various other structures that rely on agitation and oxidationof a cleansing fluid to remove undesirable gases, mist, dust, fumes,smoke and/or particulate matter from a volume of gas.

The absorber 110 shown in FIG. 2 generally has an upright structurecomposed of a spray tower 114. As illustrated, the tower 114 has anupper end, a lower end, and an inlet duct 112 which forms an opening atthe perimeter of the tower 114 through which flue gases enter theabsorber 110. The source of the flue gases may be a process involvingthe combustion of fossil fuels or various industrial operations by whichundesirable gases and particulate matter are produced. The gas scrubbingoperation occurs by contacting the flue gases with an appropriatecleansing fluid that will absorb the acidic gases and particulate matterin the flue gases. Afterwards, the flue gases are permitted to escape toatmosphere through a suitable mist eliminator 124 or any other suitableapparatus known in the art.

For purposes of removing acidic gases from flue gases, the cleansingfluid is typically an alkaline slurry. Wet flue gas desulfurizationprocesses for removing sulfur dioxide from combustion gases typicallyinvolve the use of calcium-based slurries or sodium-based orammonia-based solutions. Examples of calcium-based slurries arelimestone slurries and hydrated lime slurries formed by action of wateron lime. Intimate contact between the alkaline slurry and acidic gasespresent in the flue gases, such as sulfur dioxide, hydrogen chloride andhydrogen fluoride, results in the absorption of the gases by the slurry.Thereafter, the slurry can be oxidized to cause the alkali to react withthe absorbed acidic gases to yield a useful byproduct. For example, inthe case of desulfurization where a calcium-based alkaline slurry isused to absorb sulfur dioxide, an oxygen-containing gas such as air isinjected into the fluid to oxidize aqueous sulfite (SO₃ ⁻) into sulfate(SO₄ ⁻), the latter of which will then react with calcium ions in theslurry to form gypsum (CaSO₄) as a saleable byproduct. As a result ofthis ongoing reaction, some gypsum inherently remains in suspension inthe slurry, and is therefore recycled to the tower 114 along with thealkali solids. While the above reaction is exemplary, the teachings ofthis invention are not limited to the use of calcium-based slurries in adesulfurization reaction.

As shown in FIG. 2, the slurry can be delivered to the tower 114 by oneor more banks 116 of spray headers 118. The spray headers 118 aredesigned to disperse a spray into the tower 114 that falls due to theforce of gravity countercurrently to the upwardly flowing flue gases,such that fluid droplets entrap the particulate matter and absorb theacidic gases. While spray headers 118 are shown in FIG. 2, it isforeseeable that other types of devices could be used for introducingthe slurry to the tower 114, including atomizers and trays. Furthermore,though FIG. 2 shows three banks 116 of spray headers 118, the number andplacement of the banks 116 and spray headers 118 can be readily adaptedto the design requirements of a particular application.

As also shown in FIG. 2, the absorber 110 includes a tank 122 at thelower end of the tower 114 in which the slurry is collected aftercontacting the flue gases in the tower 114. A pump 126 is fluidicallyinterconnected through an inlet 140 with the tank 122, and serves toreturn the slurry to the spray headers 118. Also shown is an overflowduct 120 connected to the tank 122, which allows removal of slurry inexcess of the amount required for the scrubbing operation. According tothis invention, the tank 122 is uniquely adapted to oxidize and agitatethe accumulated slurry in order to form a desired byproduct and maintainthe solid precipitates in suspension within the tank 122. In particular,the tank 122 of this invention is equipped with a partition 134 thatdelineates an inner region 138 and an annular-shaped outer region 136within the tank 122. As shown in FIG. 2, the partition 134 is centrallylocated in the tank 122, and has a tubular shape with a lower end beingsubmerged in the slurry within the tank 22, while the upper end of thepartition 134 projects above the surface of the slurry. Shown extendinginto the lower end of the inner region 138 is an injector 128 forinjecting an oxygen-containing gas, such as air, into the slurry forcausing forced oxidation of the slurry within the tank 122. As shown,air is delivered to the injector 128 by an aerator 144, represented hereas a blower, though other types of aerators and injectors could be used,including spargers, air lanced agitators and aspirators. In the positionshown, the partition 134 forms a physical barrier between the oxygeninjected into the tank 122 and the inlet 140 to the pump 126. As such,there is a significantly reduced tendency for froth and bubbles producedby aeration to be drawn into the pump 126.

In contrast to the prior art use of a fan agitator emersed in theslurry, the present invention is able to rely solely on the introductionof the oxygen-containing gas into the inner region 138 of the tank 122in order to maintain the solids in suspension and distribute anddissolve the oxygen in the slurry. As air is injected into the innerregion 138, the slurry within the inner region 138 froths and expands,overflows the upper end of the partition 134, degasses, and then fallsinto the outer region 136. Once in the outer region 138, the slurry iseventually recirculated back to the inner region 138 by being drawn tothe lower end of the partition 134. Advantageously, the slurry containedin the tank 122 is sufficiently agitated in this manner to insure thatthe solids in the slurry are maintained in suspension and oxygen isadequately distributed. However, the agitation level created in thismanner is restricted to the inner region 138 of the partition 134, andtherefore does not cause frothing and expansion of the slurry within theouter region 136 of the tank 122, as occurs with prior art systems.Consequently, because the capacity of the tank 122 required toaccommodate frothing of the slurry is significantly reduced, the overallheight of the absorber 110 can be reduced.

In operation, flue gases are introduced into the tower 114 through theinlet 112, while the alkaline slurry is introduced into the tower 114through the spray headers 118 so as to flow countercurrently relative tothe upwardly-flowing flue gases. Thereafter, the slurry is accumulatedin the tank 122, where an oxygen-containing gas, such as air, isinjected into the inner region 138 of the tank 122 formed by thepartition 134. In doing so, the slurry absorbs the oxygen-containing gasand expands within the inner region 138, with the result that the slurrywithin the inner region 138 is forced up and over the partition 134,where it degasses before returning to the annular-shaped outer region136 within the tank 122. The slurry within the tank 122 continues torecirculate in this manner, with a portion of the slurry being drawn bythe pump 126 from the tank 122 and returned to the spray headers 118.Because frothing and bubbles formed by aeration are primarily limited tothe inner region 138, bubbles are inhibited from being drawn into thepump 126 through the inlet 140. As gypsum is produced, a portion isremoved for either disposal or sale, while additional alkali 132 isadded to the tank 122 in order to compensate for that which has beenreacted and removed as gypsum. In the position shown, the partition 134also forms a physical barrier between the newly added alkali and theoxygen injected into the tank 122, thereby significantly reducing thetendency for sulfite binding to occur.

From the above, it can be appreciated that this invention uniquelycombines the functions of agitation and oxidization within a single unitcomposed of the partition 134 and the aerator 144. This capabilitycompletely eliminates the capital and operating costs associated withoperating a conventional aerator, such as the fan, and therefore reducesboth the initial construction costs and the daily operation andmaintenance costs of the absorber 110. Another advantage of the absorber110 is that it provides a gentler agitation than in prior art spraytowers. Gentler agitation reduces secondary nucleation of gypsumcrystals, and therefore allows the crystals to grow larger than ispossible with prior art agitators such as the fans shown in FIG. 1. As aresult, the gypsum crystals can be dewatered and dried more easily.

Yet another advantage of this invention is that the aerator 144 injectsthe oxygen-containing gas only into the inner region 138 of the tank122, rather than into the entire tank 122. As a result, the conventionalrequirement to allow for a considerable rise in the slurry height withinthe tank 122 is avoided, and the size of the tank 122 is correspondinglyreduced. The partition 134 also advantageously serves as a barrier toprevent bubbles introduced through the aerator 144 from being drawn intothe pump 126, and reduces the tendency of sulfite binding of newly addedalkali.

FIG. 3 illustrates another flue gas scrubber in the form of an absorber210 configured in accordance with the combined teachings of the presentinvention and that of co-pending and commonly assigned U.S. patentapplication Ser. No. 08/335,589 to Laslo. As illustrated, the absorber210 significantly differs from the prior art absorber 10 shown in FIG. 1and the absorber 110 of FIG. 2. Yet, in accordance with this invention,the absorber 210 encompasses the use of a partition 234 within which theslurry is oxidized and agitated solely through the operation of anaerator 244. Further-more, the absorber 210 avoids the requirement for apump to deliver the slurry to spray headers 218 within the gas-liquidcontactor 214 of the absorber 210. As a result, the construction,operational and maintenance costs of the absorber 210 are even furtherreduced as compared to the prior art absorber 10.

As with the absorber 110 of FIG. 2, the absorber 210 shown in FIG. 3generally has an upright structure composed of the gas-liquid contactor214, with an inlet duct 212 at its lower end through which flue gasesenter the absorber 210. Eventually, the flue gases are permitted toescape to atmosphere through a suitable mist eliminator 224 or any othersuitable apparatus known in the art. As before, acidic gases are removedfrom the flue gases with a cleansing fluid, which may be alkaline suchas a calcium-based slurry or a sodium or ammonia-based solution. Asshown in FIG. 3, the fluid can be delivered to the contactor 214 throughone or more spray headers 218 or other suitable devices.

As also shown in FIG. 3, the absorber 210 includes a separate tank 222in which the fluid is collected after absorbing the flue gases in thecontactor 214. In contrast to the countercurrent contact mode employedin the absorber 110 of FIG. 2, the absorber 210 of FIG. 3 operates withflue gases flowing at a much high velocity such that the cleansing fluidflows co-currently with the flue gases toward the upper end of thecontactor 214. The fluid is recovered at the upper end of the contactor214 with a disengagement device 250, such as the hydrocyclone shown orany other device capable of separating the fluid particles from thecleansed flue gas, such that the liquid particles fall out of the airstream and are eventually accumulated in the tank 222 via return line248.

As before, an overflow duct 220 is connected to the tank 222, whichallows removal of cleansing fluid in excess of the amount required forthe scrubbing operation. According to this invention, the tank 222 isalso equipped with a partition 234 that delineates an inner region 238and an annular-shaped outer region 236 within the tank 222. As with theembodiment of FIG. 2, the partition 234 is centrally located in the tank222, and an aerator 244 serves to inject air or another suitableoxygen-containing gas through a pipe 228 and into the fluid, causingforced oxidation of the fluid within the partition 234. In contrast tothe previous embodiment, the fluid level within the tank 222 andpartition 234 is at a greater elevation than the spray header 218. Inaddition, the partition 234 is equipped with an annular-shaped trough242 that collects the fluid as it flows out and over the top of thepartition 234. In effect, the trough 242 forms an integral portion ofthe inner region 238, though it is foreseeable that the trough 242 couldbe separate from the partition 234 and the inner region 238 to someextent. The trough 242 may be adapted to require the fluid to overflowits rim in order to return to the outer region 236 of the tank 222, ormay be provided with openings along its sides and/or lower surface toallow passage of the fluid to the outer region 236.

A conduit 226, fluidically interconnected with the tank 222 througheither of two inlets 240a and 240b, serves to return the slurry to thespray header 218 solely under the force of gravity. As shown, the inlet240a interconnects with the tank 222 at an elevation a distance "X"above the spray header 218, while the inlet 240b interconnects with thetrough 242 at an elevation a distance "X+Y" above the spray header 218.As such, the fluid will flow solely under the force of gravity to thespray header 218, rendering unnecessary the pump 126 shown in FIG. 2.Notably, a greater pressure head is achieved between the inlet 240b andthe spray header 218 than between the inlet 240a and the spray header218, achieving a higher flow rate to the spray header 218 through moreefficient utilization of the energy required to operate the aerator 244.Flow through the conduit 226 can be regulated to draw fluid from eitherthe trough 242 or the tank 222, or both simultaneously. Furthermore, asmall portion of the fluid can bypass the tank 222 entirely and bereturned directly to the spray header 118 through a second conduit 246in order to provide dissolved alkalinity to the oxidized fluid beingreturned from the trough 242 or tank 222.

In addition to the additional benefits and features noted above, theabsorber 210 represented in FIG. 3 has the same advantages as thatdescribed for the absorber 10 of FIG. 2.

While our invention has been described in terms of preferredembodiments, it is apparent that other forms could be adopted by oneskilled in the art, such as by incorporating the novel features of thisinvention within flue gas absorbers that structurally differ from thoseshown in the Figures, and by altering the shape and construction of thepartitions 134 and 234. Accordingly, the scope of our invention is to belimited only by the following claims.

What is claimed is:
 1. A flue gas scrubbing method comprising the stepsof:introducing flue gases comprising acidic gases into a passage;introducing an alkali-containing fluid into the passage such that thealkali-containing fluid removes the acidic gases from the flue gases;accumulating the alkali-containing fluid in a tank located below saidpassage, said tank being in open communication with the passage;injecting an oxygen-containing gas into the tank so as to cause thealkali-containing fluid to flow up through a first region structurallydelineated within the tank and down into a second region structurallydelineated within the tank, and so as to simultaneously expand, agitateand oxidize the alkali-containing fluid within the first region prior toflowing into the second region; after returning to the second region,recirculating a first portion of the alkali-containing fluid to thefirst region; and drawing a second portion of the alkali-containingfluid from the second region of the tank and returning the secondportion of the alkali-containing fluid to the passage.
 2. A method asrecited in claim 1 wherein the alkali-containing fluid is circulated upthrough an inner region of the tank and down into an outer regionsurrounding the inner region.
 3. A flue gas scrubbing method comprisingthe steps of:introducing flue gases comprising acidic gases into apassage; introducing an alkali-containing fluid into the passage suchthat the alkali-containing fluid removes the acidic gases from the fluegases; accumulating the alkali-containing fluid in a tank; injecting anoxygen-containing gas into the tank so as to cause the alkali-containingfluid to flow up through a first region structurally delineated withinthe tank and down into a second region structurally delineated withinthe tank, and so as to simultaneously expand, agitate and oxidize thealkali-containing fluid within the first region prior to flowing intothe second region; after returning to the first region, recirculating afirst portion of the alkali-containing fluid to the first region; anddrawing a second portion of the alkali-containing fluid from the secondregion of the tank and returning the second portion of thealkali-containing fluid to the passage, wherein the second portion ofthe alkali-containing fluid is drawn from the second region of the tankat a first elevation that is above a second elevation at which thesecond portion of the alkali-containing fluid is introduced into thepassage, such that the drawing step occurs solely under the influence ofgravity to return the second portion of the alkali-containing fluid tothe passage.
 4. A method as recited in claim 3 further comprising thestep of returning to the passage a portion of the alkali-containingfluid after the alkali-containing fluid has been introduced into thepassage but prior to being accumulated in the tank.
 5. A method asrecited in claim 3 wherein the alkali-containing fluid flowsco-currently with the flue gases through the passage.
 6. A method asrecited in claim 1 wherein the alkali-containing fluid flowscounter-currently to the flue gases through the passage.
 7. A method asrecited in claim 1, wherein the alkali-containing fluid is accumulatedfrom the passage in the second region of a tank.